1. Field of the Invention
The present invention is directed to isolated thraustochytrid microorganisms as well as strains and mutants thereof. The invention is further directed to thraustochytrid biomasses, microbial oils, compositions, cultures, methods of producing the microbial oils, and methods of using the isolated thraustochytrids, biomasses, and microbial oils.
2. Background Art
Fatty acids are classified based on the length and saturation characteristics of the carbon chain. Short chain fatty acids generally have 12 carbons or less, medium chain fatty acids generally have 14 to 18 carbons, and long chain fatty acids generally have 20 or more carbons. Fatty acids are termed saturated fatty acids when no double bonds are present between the carbon atoms and are termed unsaturated fatty acids when double bonds are present. Unsaturated long chain fatty acids are monounsaturated when only one double bond is present and are polyunsaturated when more than one double bond is present.
Polyunsaturated fatty acids (PUFAs) are classified based on the position of the first double bond from the methyl end of the fatty acid: omega-3 (n-3) fatty acids contain a first double bond at the third carbon, while omega-6 (n-6) fatty acids contain a first double bond at the sixth carbon. For example, docosahexaenoic acid (“DHA”) is an omega-3 long chain polyunsaturated fatty acid (LC-PUFA) with a chain length of 22 carbons and 6 double bonds, often designated as “22:6 n-3.” Other omega-3 LC-PUFAs include eicosapentaenoic acid (“EPA”), designated as “20:5 n-3,” and omega-3 docosapentaenoic acid (“DPA n-3”), designated as “22:5 n-3.” DHA and EPA have been termed “essential” fatty acids. Omega-6 LC-PUFAs include arachidonic acid (“ARA”), designated as “20:4 n-6,” and omega-6 docosapentaenoic acid (“DPA n-6”), designated as “22:5 n-6.”
Omega-3, fatty acids are biologically important molecules that affect cellular physiology due to their presence in cell membranes, regulate production and gene expression of biologically active compounds, and serve as biosynthetic substrates. Roche, H. M., Proc. Nutr. Soc. 58: 397-401 (1999). DHA, for example, accounts for approximately 15%-20% of lipids in the human cerebral cortex, 30%-60% of lipids in the retina, is concentrated in the testes and sperm, and is an important component of breast milk. Jean-Pascal Bergé & Gilles Barnathan, Fatty Acids from Lipids of Marine Organisms: Molecular Biodiversity, Roles as Biomarkers, Biologically Active Compounds, and Economical Aspects, in Marine Biotechnology I 49 (T. Scheper, ed., 2005). DHA accounts for up to 97% of the omega-3 fatty acids in the brain and up to 93% of the omega-3 fatty acids in the retina. Moreover, DHA is essential for both fetal and infant development as well as maintenance of cognitive functions in adults. Id. Omega-3, fatty acids, including DHA and EPA, also possess anti-inflammatory properties. See, e.g., Id. and Simopoulos, A. P., J. Am. Coll. Nutr. 21: 495-595 (2002). Because omega-3, fatty acids are not synthesized de novo in the human body, these fatty acids must be derived from nutritional sources.
Flaxseed oil and fish oils are considered good dietary sources of omega-3 fatty acids. Flaxseed oil contains no EPA, DHA, DPA, or ARA but rather contains linolenic acid (C18:3 n-3), a building block enabling the body to manufacture EPA. There is evidence, however, that the rate of metabolic conversion can be slow and variable, particularly among those with impaired health. Fish oils vary considerably in the type and level of fatty acid composition depending on the particular species and their diets. For example, fish raised by aquaculture tend to have a lower level of omega-3 fatty acids than those in the wild. Furthermore, fish oils carry the risk of containing environmental contaminants and can be associated with stability problems and a fishy odor or taste.
Thraustochytrids are microorganisms of the order Thraustochytriales. Thraustochytrids include members of the genus Schizochytrium, and have been recognized as an alternative source of omega-3 fatty acids, including DHA. See U.S. Pat. No. 5,130,242. Oils produced from these marine heterotrophic microorganisms often have simpler polyunsaturated fatty acid profiles than corresponding fish or microalgal oils. Lewis, T. E., Mar. Biotechnol. 1: 580-587 (1999). Strains of thraustrochytrid species have been reported to produce omega-3 fatty acids as a high percentage of the total fatty acids produced by the organisms. U.S. Pat. No. 5,130,242; Huang, J. et al., J. Am. Oil. Chem. Soc. 78: 605-610 (2001); Huang, J. et al., Mar. Biotechnol. 5: 450-457 (2003). However, isolated thraustochytrids vary in the identity and amounts of LC-PUFAs produced, such that some previously described strains can have undesirable levels of omega-6 fatty acids and/or can demonstrate low productivity in culture. As such, a continuing need exists for the isolation of thraustochytrids demonstrating high productivity and desirable LC-PUFA profiles.